The subject matter of the present invention relates generally to the manufacture of hollow ceramic articles, such as beams, by molding them as one piece using a flexible mold and molding mandrels and, in particular, to a method and apparatus for the manufacture of hollow ceramic beams using multiple molding mandrels. The mandrels are placed within a flexible mold and mounted in a predetermined spaced relationship so that a mixture of ceramic-forming powder poured into the mold fills the mold including the spaces between the mandrels and the mandrels are allowed to move laterally during pressing. By pressing the mold in an isostatic press to form a compressed body of such ceramic powder in the shape of a hollow beam, internal support partitions are formed between the mandrels as part of the compressed body. After the mandrels are removed from the compressed body, such body is fired to sinter the ceramic powder into a hollow ceramic beam having internal support partitions formed integral with the outer wall of the beam. These support partitions prevent the walls from sagging downward due to gravity during firing (hereafter called "slumpage").
Isostatic pressing of a mixture of ceramic-forming powder in a flexible mold of rubber to provide a pressed body of powder which is subsequently machined to the proper dimensions and then fired to sinter the body into a ceramic article has been used to manufacture solid and hollow ceramic articles as described in Elements of Ceramics by F.H. Norton, pages 109-110, 2nd edition, published 1974 by Addison-Wesley Publishing Co. of Menlo Park, California.
As described in U.S. Pat. No. 3,824,051 of Van Leemput issued July 16, 1974, it has previously been the practice in the manufacture of hollow ceramic articles by pressing in a mold to employ a single molding mandrel which is mounted within a flexible mold to form a compressed body of ceramic-forming material in an isostatic press. When hollow ceramic beams are manufactured in this manner to provide a hollow compressed body having no internal partitions that is fired in a kiln to sinter the ceramic, the high temperature causes slumpage of the body which tends to bend the walls of the beam downwardly, resulting in deformity of the beam. Heretofore, it has been proposed to provide separate ceramic support plates which are positioned within the hollow pressed body before sintering in order to prevent collapse of the wall during firing of the pressed body. However, this has the disadvantage that there may be a discontinuity between the density of the internal support plate and that of the inside surface of the beam at its interface with the plate. The inside surface of a hollow pressed beam may have a reduced density from that of a tri-axially pressed solid body because of the effect of the friction forces between the hollow ceramic body and the mandrel. These friction forces include compression of the body vertically along the length of the mandrel, thus producing a bi-axial horizontal compression which may not produce the high densities achieved with tri-axial compression. The amount of shrinkage that a ceramic body undergoes is dependent on density among other things. Since the internal ceramic support plates adhere to the walls of the ceramic beam, the above-described difference in density causes a difference in shrinkage, which will result in strain in the internal ceramic support plate and the hollow pressed beam. The resulting tensile strain may be sufficient to cause failure during the sintering process.
In an effort to avoid discontinuities in beam density and to improve manufacturability it was decided to utilize multiple parallel mandrels. These mandrels would be placed inside the press bag in such a position as to allow ceramic body to flow with the gap between two mandrels to form the desired support web. However, difficulties were experienced when this was attempted, including cracks in the isostatically pressed beams. It appears that these cracks were the result of non-uniform web compression which caused bowing of the mandrels during compression. Upon release of the isostatic pressure, the mandrels attempt to elastically return to their prepressed shape, resulting in strain in the mandrels and in the pressed beam body. The tensile strain is often sufficient to fracture the pressed beam body. The non-uniform compression of the web between the mandrels is caused by more additional ceramic powder flowing into the web area near the ends of the beam than near the center of the beam once isostatic compression has begun. The web portions near the ends of the mandrels are exposed to compression forces parallel to the mandrels as well as compression forces normal to the mandrels. However, friction between the mandrels and the ceramic body restricts the compressive forces parallel to the mandrels in the web near the center of the mandrels resulting in only normal compressive forces near the center of the web.